1. Field of the Invention
The present invention relates to a battery charger for rapidly charging a secondary battery
2. Description of the Prior Art
A conventional rapid battery charger as described in Japanese Laid-Open Patent Publication No. HEI-6-121468 supplies a large current to a secondary battery during the initial stage of charging. During the charging process, a CPU periodically sets the charging current to zero (off) and measures the open-circuit voltage of the secondary battery. The open-circuit voltage is then compared with a previously set reference voltage. If the open-circuit voltage is determined to exceed the reference voltage, then the current used for charging is changed to a lower value, signifying a progression in the charging process.
This technology reduces the current before gas is produced, allowing the battery to be charged to a high voltage level while preventing the generation of gas which may otherwise be generated if the battery is charged with a large current. Gas is produced when the voltage across the secondary battery exceeds a certain value.
When a secondary battery is being charged, the voltage across the battery increases due to the current applied to the battery and also to increases in the amount of electrical charges stored in the battery. The voltage eventually reaches a point at which gas is generated within the battery. In rapid charging, overvoltage also rises considerably due to the large current that is flowed through the secondary battery. To reduce this overvoltage, the current has conventionally been reduced during the charging process before gas is generated.
With another type of battery charger, the open-circuit voltage of the secondary battery is periodically measured during the charging process and compared with a reference voltage. The charging process is terminated at the point that the voltage exceeds the reference voltage.
The open-circuit voltage is measured when current is not being applied to the secondary battery in order to avoid ohmic dissipation from resistances in the battery leads and terminal contacts and from internal resistance of the battery.
A battery charger as described above, however, requires that a different voltage be set for each type of secondary battery, because the overvoltages that result from a charging current are not the same for every type of secondary battery. Such a requirement makes operating the battery charger more complicated.
If the reference voltage is fixed at a certain value designed for one type of battery but used to represent all types of batteries the expected charging results will not be obtained with many batteries, because charging characteristics vary widely from battery to battery. Fixing the reference voltage in this way may result in low charges, invite the generation of excessive gas, and the like.